In the conventional way, the master cylinder is full of brake fluid and equiped with a primary piston intended to receive an actuating force composed of an input force and of a boost force both acting in an axial direction, so as to cause an increase in pressure in a primary hydraulic circuit, and with a secondary hydraulic piston subjected to the hydraulic pressure in the primary hydraulic circuit and causing an increase in pressure in a secondary hydraulic circuit.
Also, in the conventional way, the pneumatic booster can be controlled by the application of the input force to a control rod controlling the opening of a valve in order to exert the actuating force on the primary piston of the master cylinder, the booster including a rigid casing divided in leaktight fashion into two chambers by means of a moving partition which can be acted upon by a difference in pressure between the two chambers resulting from the opening of the valve and can drive a pneumatic piston, which can move with respect to the casing, carrying the valve, the input force being transmitted via a reaction disc against which the pneumatic piston also rests in order to impart at least some of the boost force to it.
A device of this type is well known in the prior art and is described, for example, in document U.S. Pat. No. 4,491,058.
In parallel, boosted braking devices have been developed in which the reaction exerted on the control rod is not supplied mechanically by a reaction disc, but by the hydraulic pressure prevailing in the master cylinder.
In these devices, she main hydraulic piston of the master cylinder itself includes a hollow moving cylinder communicating with the master cylinder and receiving at least some of the boost force, and inside which there slides, in leaktight fashion and in the axial direction, a secondary hydraulic piston which can receive at least the input force, elastic means exerting an elastic force between the secondary hydraulic piston and the moving cylinder and urging the secondary hydraulic piston towards the master cylinder, at least one opening being made in the moving cylinder to make the inside of the latter communicate with the inside of the master cylinder.
Such a device is described, for example, in document FR-A-2,658,466.
These devices with hydraulic reaction have the main advantage that irrespective of the intensity of the braking action or the rate of application of the input force, their characteristic operating curve, namely the curve giving the pressure in the master cylinder as a function of the intensity of the input force on the booster, is unchanged.
However, in the event of failure in the hydraulic circuit between the master cylinder and the wheel brakes, the pressure inside the primary or secondary hydraulic circuit cannot be established. In both cases this results in an increase in the travel of the brake pedal in order to obtain an increase in the hydraulic pressure in the intact circuit. However, if this failure affects the primary hydraulic circuit, this additionally results in a complete loss of reaction felt by the driver on the brake pedal, which means that he can no longer gauge the braking force except by gauging the travel of the brake pedal, which travel is itself already disturbed.
Attempts have already been made to solve this problem, for example in documents FR-A-1,475,049 or U.S. Pat. No. 3,470,695. These documents envisage the use of the hydraulic pressure in the secondary circuit, assumed to be intact, to provide the desired reaction on the brake pedal. Aside from the fact that they result in complicated embodiments, they require the presence of a third piston sliding in leaktight fashion in the secondary piston. Such an arrangement is prohibited by current legislation, because it introduces a risk of leakage between the primary and secondary circuits, which would not be detectable under normal operating conditions, that is to say unless there were a failure, which conditions are usually satisfied. In addition, the boosted braking systems in these documents do not display the characteristic of a "jump" at the beginning of a braking action, which characteristic is now required by motor vehicle manufacturers to improve the feel experienced by the driver when he actuates the brake device.
The present invention falls within this context and its object is to propose a boosted braking device with hydraulic reaction, which makes the driver, under all operating circumstances, feel a reaction on the brake pedal even in the event of failure of the primary hydraulic circuit, without requiring a complicated device, and which is therefore of low cost while having an operation which is reliable under all circumstances, the leaktightness between the primary and secondary circuits complying with legislation.
To this end, the present invention proposes a boosted braking device for a motor vehicle, comprising on the one hand a master cylinder full of brake fluid and equipped with a primary piston intended to receive an actuating force composed of an input force and of a boost force both acting in an axial direction, so as to cause an increase in pressure in a primary hydraulic circuit, and with a secondary hydraulic piston subjected to the hydraulic pressure in the primary hydraulic circuit and causing an increase in pressure in a secondary hydraulic circuit, and on the other hand a pneumatic booster which can be controlled by the application of the input force to a control rod integral with a plunger controlling the opening of a three-way valve in order to exert the actuating force on the primary piston, the booster including a rigid casing divided in leaktight fashion into at least two chambers by means of at least one moving partition which can be acted upon by a difference in pressure between the two chambers resulting from the opening of the three-way valve and can drive a pneumatic piston, which can move with respect to the casing, carrying the three-way valve and contributing at least to transmitting the boost force, the primary piston of the master cylinder itself including a hollow moving cylinder communicating with the primary hydraulic circuit of the master cylinder, receiving at least some of the boost force, and inside which there slides, in leaktight fashion and in the axial direction, a reaction piston which can receive at least the input force, first elastic means exerting a first elastic force between the reaction piston and the moving cylinder and urging the reaction piston towards the master cylinder, at least one opening being made in the moving cylinder to make the inside of the latter communicate with the primary hydraulic circuit.
Such a boosted braking device is known, for example, from document EP-B-0,662,894.